Dispatch communication systems are known. Such systems typically use frequency pairs (i.e., a transmit frequency and a receive frequency) that are assigned by a resource controller for transmitting and receiving messages. The resource controller assigns the frequency pair upon receiving a request for service from a communications unit or from a telephone interconnect party. Upon receiving the assignment data, the communication unit(s) involved tune to the appropriate frequencies and two-way communication can then occur.
Dispatch systems, unlike cellular telephone, are typically single site systems. While it is common for there to be large geographic areas covered by multiple adjacent, but independent dispatch systems, such systems do not provide for call hand-off when a user leaves the service area of one system and enters the service area of another system. Upon loss of contact, the communication unit which has left a dispatch system's service area may no longer participate in communications which were in progress at the time of the loss of contact.
In a group conversation, the unit lost is no longer part of the group conversation. However, in the case of a telephone interconnect conversation between a single communication unit and a land-line connected party, the conversation is interrupted when the communication unit leaves the service coverage area, although the connection between the land-line connected party and the trunked dispatch system where the call was set-up is still intact.
In the case of interrupted telephone interconnect conversations, the call must be re-initiated between the communication unit which has moved to the service coverage area of a different dispatch system, and the land-line party. However, if the call was initiated by the land-line party, the communication unit may not have a telephone number to dial-back the land-line party from the new dispatch system. Further, the land-line party may not be immediately aware of the loss of communications, resulting in the likelihood that the communication unit dialing back the land-line party will find the line busy. Further, the land-line party may not have knowledge of the location of the new service coverage area that the communication unit has entered following the loss of contact, thus inhibiting the land-line party from being able to re-initiate the call.
The requirement that a dispatch or interconnect user must re-establish contact every time a communication unit leaves a particular coverage area even though it may have moved in the service coverage area of an adjacent trunked system is an impediment to effective mobile communications. A need therefore exists to transfer calls to adjacent trunked systems without the need to re-initiate the call sequence.